1. Field of the Invention
The present invention relates to a method of applying glass layers, which may or may not be doped, to the interior of a substrate tube by means of a chemical vapour deposition (CVD) technique, using a reactive gas mixture, in order to obtain a preform that exhibits a precisely defined refractive index profile.
2. Description of the Related Art
A method of this kind is known per se from U.S. Pat. No. 4,292,341. According to said US patent, efficient transmission of optical wave energy along an optical fiber is achieved by grading the refractive index of the fiber core. Thus, accurate knowledge of the index profile is necessary in order to assess a fiber""s transmission properties. To this end, various techniques have been developed for measuring the index profile of optical fibers and optical fiber preforms. According to the method known from the aforesaid US patent, a precursor, in the form of a gas containing glass forming material and suitable index modifying dopants, is caused to flow into a substrate tube. The substrate tube is then heated, causing a layer of glass to be deposited on the inner surface of the tube. Thus the thickness of each deposited layer, and the concentration of dopants within each layer can be considered to be functions of a number of parameters including the temperature of the hot zone created within the tube, the speed with which the hot zone moves along the tube, and the concentrations of the glass forming material and the index modifying dopants within the gas being fed to be tube. Thus the exact concentrations in the gaseous mixture being fed to the substrate tube are accurately measured and carefully checked during manufacture of the preform. To this end the gaseous components, for example GeCl4 and SiCl4, are illuminated with ultraviolet radiation, after which the intensity of the radiation energy from said gaseous material is measured, which measurement is compared with a reference signal, whereby a control signal is generated in response to this comparison, which control signal causes the concentration of GeCl4 and/or SiCl4 in the gaseous material to change. Such a change will only take place until the difference between the reference signal and the control signal has reached value zero. When using such a method, it will be possible to manufacture a preform exhibiting a precisely computed refractive index profile. One drawback of a technique of this kind is the fact that in practice the measurements whereby ultraviolet radiation is used are not accurate and reproducible to a sufficient degree, so that a preform is obtained which exhibits an unacceptable variation in comparison with the desired refractive index profile of the preform to be manufactured. In addition, such a method is only suitable for supplying one dopant, because generally two or more dopants cannot be distinguished from each other to a sufficient degree and because they produce interference upon being illuminated with ultraviolet radiation.
A method of this kind is also known from U.S. Pat. No. 4,161,656, wherein the concentration and distribution of index-modifying dopants is determined by illuminating a length of the fiber or fiber preform with ultraviolet radiation. A drawback that occurs thereby is that such a measurement does not meet the requirements that are currently made with regard to accuracy. In addition to that, exact feedback between the process conditions during deposition and the measured values is difficult to realize in practice, so that the profile that is eventually obtained exhibits differences with the intended profile.
In one embodiment, a method of applying glass layers, which may or may not be doped, to the interior of a substrate tube by means of a chemical vapour deposition (CVD) technique, using a reactive gas mixture, in order to obtain a preform that exhibits a precisely defined refractive index profile, includes but is not limited to: a) determining the desired refractive index profile of the preform to be manufactured, b)precisely adjusting both the composition and the supply rate of the reactive gas mixture for manufacturing the intended preform, in conformity with the refractive index profile determined in step a), c) introducing the reactive gas mixture, under the conditions adjusted in step b), into the substrate tube and effecting a reaction therein so as to achieve deposition of the glass-forming oxides onto the interior of the substrate tube, d)collapsing the substrate tube obtained from the deposition process of step c) into a preform and subsequently subjecting said preform to a refractive index profile measurement, e) comparing the refractive index profile determined in step a) with the refractive index profile measured in step d), and f) correcting the differences in refractive index profiles measured in step e) by adapting the composition of the reactive gas mixture as a function of time during the subsequent deposition process.
Accordingly, an embodiment of the present invention provides a method of manufacturing a preform exhibiting a precisely defined refractive index profile by means of a chemical vapour deposition (CVD) technique.
Another embodiment of the present invention provides a method of manufacturing a preform by means of a chemical vapour deposition (CVD) technique wherein two or more dopants can be used for increasing or decreasing the refractive index value in a continuous deposition process.
Another embodiment of the present invention provides a method of manufacturing a preform by means of a chemical vapour deposition (CVD) technique wherein a preform exhibiting any desired refractive index profile can be obtained in a reproducible manner.
According to an embodiment of the present invention, the method referred to in the introduction is characterized in that said method comprises the following steps:
a) determining the desired refractive index profile of the preform to be manufactured,
b) precisely adjusting both the composition and the supply rate of the reactive gas mixture for manufacturing the intended preform, in conformity with the refractive index profile determined in step a),
c) introducing the reactive gas mixture, under the conditions adjusted in step b), into the substrate tube and effecting a reaction therein so as to achieve deposition of the glass-forming oxides onto the interior of the substrate tube,
d) collapsing the substrate tube obtained from the deposition process of step c) into a preform and subsequently subjecting said preform to a refractive index profile measurement,
e) comparing the refractive index profile determined in step a) with the refractive index profile measured in step d), and
f) correcting the differences in refractive index profiles measured in step e) by adapting the composition of the reactive gas mixture as a function of time during the subsequent deposition process.
Using the aforesaid steps a)-f), it is possible to obtain precisely defined refractive index profiles in a preform whereby there is feedback of the refractive index profile measurement in the final preform and the gas dosage used in the deposition process. The results of the refractive index profile measurement obtained in step d) are used for adjusting the production process that takes place in step c) for manufacturing the preform. On the basis of the refractive index profile measurement obtained in step d) it is thus possible according to the present invention to determine the extent to which the process, in particular the composition of the reactive gas mixture, is to be adjusted in order to approach the refractive index profile desired in step a) as best as possible. After both the composition and the supply rate have been precisely adjusted according to step b) of the present invention, the deposition process according to step c) is carried out. Once the deposition process according to step c) has been terminated, the preform thus obtained is subjected to a refractive index profile measurement in step d). The refractive index profile measured in step d) is then compared with the refractive index profile determined in step a), after which a correction of the differences in the refractive index profiles measured in step e) may take place in step f) by adapting the composition of the reactive gas mixture thereto as a function of time during the subsequent deposition process. It should be understood that the correction carried out in step f) may require that the composition of the reactive gas mixture be adapted continuously during the subsequent deposition process. If the differences in refractive index profiles that are measured in step e) are acceptable within specific tolerances, however, no correction of the composition of the reactive gas mixture will take place during the subsequent deposition process, however. Correction will only take place when the differences measured in step e) exceed a specific tolerance range.
Since it has become apparent that the dopants provided in step c) are capable of diffusing from the inner layer of the substrate tube during collapsing of the substrate tube into a preform or a massive bar, the refractive, index profile is measured after collapsing in the present invention, so that adjustments may be made in connection with this phenomenon.
A suitable method for determining the refractive index profile preferably employs a laser source, which scans the preform perpendicularly to the longitudinal axis thereof. As a result of the geometry and the refractive index pattern in the preform, the laser beam is deflected. The deflected laser beam is measured and processed into a refractive index profile of the preform. A suitable instrument for determining the refractive index profile is the Preform Analyzer, of GN nettest, U.S.A.
According to a special embodiment of the present invention, it is preferable to maintain the overall volumetric deposition rate of the glass-forming oxides onto the interior of the substrate tube at a substantially constant level during the deposition process. The term xe2x80x9coverall volumetric deposition ratexe2x80x9d is understood to mean the deposition rate both of the glass-forming oxides and of the gaseous precursors or dopants for increasing or decreasing the refractive index values. A silica glass tube is preferred as a suitable substrate tube.
The use of a constant overall volumetric deposition rate during the deposition of the glass-forming oxides, possibly supplemented with one or more dopants, onto the interior of the substrate tube is in particular advantageous since the point in time at which a layer is deposited onto the interior of the substrate tube, which point in time corresponds to a specific radial position in the preform, is exactly fixed in such a situation. After all, once it is exactly known how much volume per unit time is built in, it is in principle possible to establish a correct relation between the radial position in the preform and the time at which the deposition of the glass-forming layers of oxide, possibly supplemented with one or more dopants, takes place with a gradually changing refractive index. Thus it is precisely known at what point in time correction according to step f) is desirable.
With preforms exhibiting complex refractive index profiles it is preferable to maintain the overall volumetric deposition rate of the glass-forming oxides onto the interior of the substrate tube, at a substantially constant level within a specific refractive index profile range during the deposition process. After all, with such complex profiles different ranges can be distinguished, for which different deposition conditions can be selected, which conditions are constant for each individual range, however.
The reactive gas mixture used in the present invention preferably comprises SiCl4 and O2, which reactive gas mixture may or may not contain one or more gaseous precursors or dopants for increasing or decreasing the refractive index value in comparison with the SiO2 formed from SiCl4 and O2.
One or more compounds selected from the group consisting of P2O5, TiO2, ZrO2, SnO2, GeO2, N, Al2O3 are used as a suitable gaseous precursor or dopant for increasing the refractive index value.
The use of such compounds or dopants is in particular advantageous in view of the possibility of exact dosage thereof, so that it is possible to obtain any desired refractive index profile.
According to the present invention it is preferable to use one or more compounds selected from the group consisting of B2O3 and F for decreasing the refractive index value during the deposition process.
The method according to the present invention is in particular suitable for the deposition of glass-forming oxides, wherein step c) is carried out while using an internal plasma or an external heat source.
The present invention can in particular be used for manufacturing preforms exhibiting a refractive index profile that changes gradually as a function of the radius of the preform. Since the composition of the reactive mixture can be varied at any time during the deposition process, it is possible to obtain a gradually changing refractive index profile, which profile precisely conforms to the intended profile.
In order to enhance the accuracy of the refractive index profile measurement as carried out in step d), it is furthermore preferable to measure the preform obtained in step c) at a number of different longitudinal positions, possibly at a number of different angles, and to subsequently average the values thus obtained.
In order to enhance the accuracy of the production process, it is furthermore preferable to average the refractive index profile measurement on the basis of the results of two or more separately manufactured preforms.